1. Field of the Invention
The present invention relates to a sheet post-processing device which is enhanced in sheet processing efficiency by preventing a support member from making an excess motion, specifically, a sheet post-processing device in which a pair of support members for supporting a sheet to be processed and putting the sheet to one side where the sheet is to be processed travel only a short distance to a sheet releasing position where the processed sheet is to be released. The present invention also relates to an image forming apparatus having this sheet post-processing device.
2. Related Background Art
Some of conventional image forming apparatuses for forming an image on a sheet, such as copying machines, printers, fax machines, and multifunction machines having these in combination, have sheet post-processing devices for processing a sheet on which an image has been formed. There are a variety of sheet post-processing devices including one for binding sheets and a punching device for punching a hole in a sheet.
Some of sheet post-processing devices are provided with intermediate stacking units and stacking trays. An intermediate stacking unit has slide guides which are a pair of support members movable in the direction orthogonal to the sheet conveying direction. Using the pair of slide guides, the intermediate stacking unit holds a sheet sent from the main body of an image forming apparatus by the sheet's side edges alone. A stacking tray is positioned below the intermediate stacking unit to catch a sheet, or a bundle of sheets, dropped from the intermediate stacking unit. To summarize, in a sheet post-processing device having an intermediate stacking unit and a stacking tray, a pair of slide guides receive sheets discharged from a sheet discharge port, align the sheets widthwise into a neat pile (so that the sheet edges along the sheet conveying direction are flush with one another), and position the stack of sheets in a binding process position to bind the sheets with a measure for binding a bundle of sheets, for example, a stapler. Thereafter, the sheet post-processing device opens the pair of slide guides to let the bundle of sheets drop onto the stacking tray. If the sheet post-processing device is a punching device, the pair of slide guides can punch a hole not only in a stack of sheets but also in a single sheet by approaching a sheet from the sides of the sheet to position the sheet at a given position.
When sheets are not to be bound, there is no need for the intermediate stacking unit to intercept sheets discharged from the discharge port and accordingly, the sheet post-processing device let sheets drop from the discharge port directly on the stacking tray by opening the pair of slide guides to a point where the slide guides do not in interfere with the falling sheets.
A low-cost type sheet post-processing device has a stapler fixed as well as one of slide guides that is on the stapler side while the other slide guide is moved alone. Accordingly, in a low-cost type sheet post-processing device, sheets are discharged and delivered to a pair of slide guides of an intermediate stacking unit, the stapler side slide guide is used as the reference in moving the other slide guide to push the sheets against the reference slide guide, to thereby align and position the stack of sheets. Thereafter, the sheet post-processing device uses the stapler to bind the stack of sheets and moves the other slide guide to a retraction position. As a result, the bundle of sheets drops through a gap between the opened slide guides onto a stacking tray.
However, in such conventional sheet post-processing device, sheets delivered without receiving post processing (hereinafter each of these sheets is referred to as single sheet) and sheets delivered after receiving post processing are stacked at different positions in the sheet width direction on the stacking tray. The difference in stacking position upon delivery is caused by the fact that a sheet delivered without receiving post processing is discharged from the main body of an image forming apparatus, centered side-to-side on a conveying path, onto the stacking tray whereas a sheet to be delivered after receiving post processing is discharged from the main body of the image forming apparatus, moved for aligning and positioning in the width direction to the stapler side slide guide, which serves as the reference, to be delivered to the stacking tray in a different position from when the sheet has left the main body of the image forming apparatus.
As described, a sheet delivered without receiving post processing and a sheet delivered after receiving post processing are different from each other in delivery position in the sheet width direction. Therefore, the slide guide has to make an excess motion to let processed sheets drop onto the stacking tray.
To elaborate, a sheet to be delivered without receiving post processing is delivered at the position indicated by the solid line in FIG. 20 whereas a sheet to be delivered after receiving post processing is delivered at the position indicated by the broken line. The left side slide guide is therefore designed to move to a position that does not interfere with a sheet delivered at the position indicated by the solid line. Even when a sheet is delivered at the position indicated by the broken line, the left side slide guide moves to the set position.
A portion denoted by reference numeral 340 in FIG. 20 is a sheet holding portion according to an embodiment of the present invention. The sheet holding portion 340 is shown for ease of comparison between prior art and the embodiment of the present invention. A sheet is delivered downward from an upper part of the drawing.
As described above, in this type of conventional sheet post-processing device, the unnecessary motion of the slide guide leads to poor sheet processing efficiency.
Furthermore, an image forming apparatus having this type of sheet post-processing device is low in image forming efficiency since an image is formed on a sheet at a rate dictated by the sheet processing efficiency of the sheet post-processing device.